1. Field of the Invention
The present invention relates to a wire electric discharge machining method for a poor conductive material such as a solar cell silicon, and a semiconductor wafer manufacturing method and a solar battery cell manufacturing method which are based on the wire electric discharge method.
2. Description of the Related Art
In the case where wire electric discharge is applied to a low volume resistance material such as a die material, deionization water is used as a machining fluid, and a power supply that is high in current peak value and short in pulse width is generally used as the machining conditions (see Electric Machining Journal Vo. 34, No. 75, 2000, lines 26 to 28 on left column of page 15). As a result, the discharge state is stabilized, and a high machining speed is obtained. The above machining conditions are suitable for “rough machining” because of its high speed property (This is also called “first cut” or “rough cut.” On the contrary, a machining that generates the discharge of a small energy while traveling a wire so as to skim over a machined surface to remove a small amount of the workpiece on the machined surface, to thereby improve a surface roughness in the machined surface is called “finishing”). However, when the above so-called rough machining conditions of the high peak and the short pulse current are applied to machining of the high volume resistivity material such as the solar cell silicon, which is equal to or higher than about 0.5 Ω·cm, the break of the wire frequently occurs, thereby disenabling the machining.
Also, there have been reported an example in which a low peak current condition such as 0.1 A is applied to the slice machining of a silicon material having a relatively low volume resistivity of about 10−2 Ω·cm, which is used for epitaxial wafer (see JP 09-248719 A, lines 2 to 9 of page 4), and an example in which the machining conditions such as a long pulse width (5 μsec to several tens μsec) and a low peak current (22 A or lower) are applied to the slice machining, and a deionization water is used as a machining fluid to conduct the discharge machining (see Electric Machining Journal Vo. 34, No. 75, 2000, lines 4 to 5 on left column of page 16, lines 7 to 24 on right column of the same page). However, there has been presumed that it is difficult to conduct the above discharge machining on the high volume resistivity material that is equal to or higher than about 0.5 Ω·cm such as the solar cell silicon material (see Electric Machining Journal Vo. 30, No. 65, 1996, lines 11 to 15 on left column of page 11, and FIG. 2).
On the other hand, in the wire discharge machining for the insulating material, decomposed carbon that has been produced by thermal action of the discharge machining in an oil machining fluid is attached onto the surface of the workpiece, and discharge is continued by using the electric conductivity of the attached carbon (for example, see JP 09-253935 A, summary). There has been presumed that it is difficult to machine the high volume resistivity material such as the solar cell silicon, which is equal to or higher than about 0.5 Ω·cm by methods other than the machining method conforming to the insulating material.
In the method where the decomposed carbon that has been produced by thermal action of the discharge is attached onto the surface of the workpiece, and discharge is continued by using the electric conductivity to machine the workpiece, the machining speed is low, and its practical use is limited. Also, in the above method, instead of the deionization water that is generally used in the wire discharge machining, the oil machining fluid is used, so a user becomes strained in the fire prevention, the treatment of the machining fluid complement, the environmental protection, and the like.